Tape automated bonding (TAB) is a common methodology used for the interconnection of integrated circuits or semiconductor devices. This method of interconnection employs a pattern of conductors which is typically carried upon and supported by a flexible, insulative, polymeric substrate film. This pattern of conductors is typically formed from chemically etched copper foil, and the substrate is typically a polyimide, polyester, or glass fiber reinforced film. The tips of the pattern of conductors, also called lead tips, are bonded to bonding pads on the active surface of a semiconductor die using either a bump bonding or a bumpless bonding process. This process is generically called inner-lead-bonding.
After bonding, the active surface of the semiconductor die is usually coated with a protective polymeric covering to prevent mechanical damage to the semiconductor die, to reduce soft errors caused by incident ionizing radiation, and to provide environmental protection from corrosive environments in which the semiconductor device may be used. The polymeric material is usually an epoxy or silicone which can be dispensed in either a glob top or a die coat process, which will be generically referred to as an encapsulation process.
In the glob top process, the polymeric glob top material is often solvent-free and quite viscous, and the resulting coating tends to be quite thick. Although overall die coverage is good, the material flow underneath the leads can be poor which typically results in voids underneath the leads. When a glob top material is used, the large volume of material and the coefficient of thermal expansion mismatch between the silicon semiconductor die and the glob top material can result in the cracking of a large semiconductor die. Thus, glob top is not the preferred process for TAB encapsulation.
Typical die coat material contains a solvent and has a lower viscosity than glob top material. Due to its lower viscosity, the die coat material flows well underneath the leads and around the inner lead bonds to cover the active semiconductor die surface. However, because of the dispensing process and the fluid nature of the die coat material, a convex or concave meniscus is formed at the outer surface of the coating, depending upon the quantity of the material dispensed on the die. Furthermore, a slight "pull back" occurs when the die coat material is cured, which results in an incomplete coverage of the device. Additionally, a thin coating of material results at the edges of the semiconductor die, precisely where a thick covering is desired. The reason why a thick polymer coating is desirable at the die edges is that bond pad metallization is typically placed near the die edges. Corrosion of the die typically starts at the edges, near the corners, due to the thin polymer coating or lack of polymer coating at the edges.
FIG. 1 illustrates a typical cross section of a partially shown encapsulated TAB die 10 as known in the prior art. A semiconductor die 12 is coated with an encapsulant 14 through a region of the die 12 that does not have an overlying TAB lead. Encapsulant 14 shows marked thinning at the die edge region 16 due to surface tension effects. If the quantity of encapsulant applied to the active surface 18 of the semiconductor die 12 exceeds a critical volume, or if the corner 20 of the die 12 has a chipped or other high surface energy region, the encapsulant 14 is typically predisposed to overflow the corner 20, which results in depositing encapsulant 22 on the die edge 24 and encapsulant 26 on the inactive surface 28 of the die 12.
FIG. 2 illustrates an additional view of the encapsulated TAB die 10 of FIG. 1 through a different cross-sectional plane cutting through a TAB lead 30, with an inner-lead-bond 31 to a bump 32. The TAB lead 30 has a downset region 34 which bends the lead 30 away from the semiconductor die 12, allowing a portion 36 of the encapsulant to fill under the downset region 34 of the TAB lead 30. Due to flow constraints imposed by the proximity of the downset region 34 to the die 12, die face voids 38 can result near the active surface 18 of the die. These voids 38 act as reservoirs into which moisture can condense and lead to corrosion of die circuitry contacted by the water. Voids 38 can also weaken the adhesive attachment of the TAB lead 30 to the active surface 18 due to a reduction of material under the downset region 34 of the lead 30, and due to the stress concentration effect induced by the voids 38. Higher levels of stress are thus transferred to the metallurgical inner-lead-bond 31. A portion 40 of the encapsulant may also flow down the die edge 24. While this portion helps to seal the die edge 24 to prevent corrosion, this overflow is typically uncontrolled and may result in depositing encapsulant on the inactive surface 28 (as previously illustrated in FIG. 1 ) which is cosmetically unacceptable and reduces thermal dissipative capacity of the die.
In general, the current TAB encapsulation process is difficult to control due to the propensity of the polymeric material to flow onto undesired surfaces, such as die edges and backsides. Although some die edge coverage is sometimes desirable, uncontrolled and uneven die edge coverage is not generally accepted in industry. Furthermore, it is also very difficult to form an encapsulant coating which is uniform in thickness across the active surface of the die. Additionally, it is also difficult to form a coating which predictably and regularly achieves a coating thickness which exceeds the minimum required thickness.
Thus, it is desirable to provide a TAB semiconductor device having none of the aforementioned disadvantages in the polymeric coating and the attendant encapsulation process.